If you search the archive, you'll find some photos of a dissasembled Thetford that may be helpful in understanding how the unit works. I'll share my understanding and someone who has actually taken one apart can disagree. There is a circular fence where all waste goes upon flushing. Solids are held within that while liquids pass through the fence and are available for flushing. That is one reason, I suspect, why the interior of the bowl is black plastic. When you pull the handle at the dump station the solids drop through and are flushed away by the liquid contents.
I'd share a couple of thoughts based on innumerable posts and five years of use. Flush with abandon - I've never been able to convince DW that long flushing can't use up capacity. It does seem to break down the solids quicker. If you're in a site with full hookup, never leave the bottom dump handle pulled out - you need the "whoosh" of liquid to efficiently evacuate the contents. Similarly, if you're dumping the Thetford after light use, fill it with fresh water until it reads "full" to get a better emptying. We try to always set up the TM when dumping so we can flush it out with water for a better rinse. We have followed the "one Potty Toddy tablet and a small bottle of Campa Chem" for recharge and have had no problems with excessive odor no matter what the temperature. Use nothing but RV tissue which is made to break down faster. Lastly, we get about 9 user days out of the Thetford - or 4 1/2 days for the two of us. If we're only out for a three day weekend, we use the TM Thetford exclusively. If we were spending a week without full hookup, we might try to use the campground facilities more often. Remember that by allowing waste to fill the pipe below the Thetford you can stretch the capacity a bit. Ditto for not quite filling it up to the charge line when charging the system with fresh water. - camp2canoe
The pump takes in this liquid from outside of the mesh and uses it to flush.
